1. Field of the Invention
The present invention relates to a ball valve disposed in the midway of a flow path, and, more particularly, to a ball valve which regulates a fluid flow and is used as a shut-off valve in both emergency state and normal operating state.
2. Description of the Related Art
A shut-off valve of ball valve type is conventionally well known. The ball valve, constructed of a ball-like valve and its valve seat, partitions a flow path into its upstream portion and its downstream portion of the flow path, and a path is drilled in the center of the ball-like valve, to which a control rod is linked. The control rod is rotated to allow the ball valve to shut off or adjust its flow rate.
Also available is a globe valve in which a partition board is provided in parallel with a flow path to divide it into two. A valve seat is disposed on the partition board, and a ball-like valve is vertically pressed onto the seat by means of the thread portion of a control rod connected to the valve.
The shut-off valves in the prior art, however, suffer the following problems. In the above-described ball valve, when the ball valve is shifted from its full-flow state in which the path of the valve is aligned with the flow path, to its partial-flow state in which the valve rotates to a position where a partial flow is established between the upstream and the downstream of the flow path, to a shut-off state in which the flow path is completely blocked, a fluid remains confined to the path in the valve and its relief portion. When the fluid is expanded by heat, a seal portion disposed around the control rod is damaged.
A large torque is needed to rotate the control rod, since the valve is tightly sealed onto its seat. This presents a operational difficulty. In particular, in the shut-off state of the flow path, the degree of seal between the valve element and the valve seat is extremely large because the pressure of the fluid is exerted onto the valve element. Thus, the valve seat suffers a rapid wear rate.
Foreign matter such as rust tends to be jammed between the flow path end of the valve element and the valve seat. Also, foreign matter deposited on the bottom of the relief portion has been a cause of jamming.
The ball valve subjects the flow of the fluid to curved path, causing a large energy loss.
In the globe valve, since the flow of the fluid is curved in a complex path in the valve seat portion, the loss of pressure is substantial. Thus, the number of shut-off valves in a piping system had to be properly controlled. Furthermore, since the control rod uses a screw mechanism, it takes time to complete a shut-off operation. The screw mechanism is definitely reliable, but presents a problem, particularly in an emergency situation. To shut off a high pressure fluid, a large force is directly exerted onto the control rod, and a large torque is required. The globe valve is complex in structure and costly to manufacture.